1. Field of the Invention
The present invention relates to a method for operation to excavate an underdrain and a device for such operation to excavate an underdrain. More particularly, the invention relates to a method for operation to excavate an underdrain with or without filling materials being used and a device for such operation.
2. Description of the Prior Art
For a conversion from rice paddy into a field cultivation, it is essential to improve water permeability and drainage in the field. For this purpose, open drainage and underdrainage have been recommended in the field while it is known that the under drain is more efficient in watery areas; that is, the acceleration of vertical permeation of surplus water is practiced as essential means. Many of conventional underdrainage excavating devices are of the type in which the plow section of a subsoiler is attached with a shell-shaped follower member or a so-called cannonball which is pulled by the tractor deep through the ground to form an underground space which is called a "shell" underdrain.
Particularly, the underdrain is not only suited to the conversion from the rice paddy cultivation to the field cultivation but also is most effective to the soil improvement of both the rice paddy and the field.
Conventionally, a known underdrain was made by forming a space in the field ground such that residual surface water or excess moisture within the ground was vertically permeated to build an aquatic environment suited to the field or rice paddies as well as improving the quality of the wet-fields. In this case, a care must be taken to prevent the underground space from being filled up with the passage of time. Actually, however, the surface soil in the field falls through an upper opening of the slit formed by the plow of the subsoiler into the slit space with the result that said slit space has been filled up as time goes on, thus frustrating the original objective.
In a peat bog, the field surface is made of clay which tends to close or fix an open slit due to plasticity and resiliency of the soil forming the underground space, thus failing the function expected as the underdrain.
It is also proposed to bury unglazed ceramic pipes in position for collecting the vertically permeating water but such pipes are merely connected in series. Therefore, the pipes can be disconnected as a result of aging to such a degree that sufficient drainage was not effected, thus making the service life of the underdrain short.